I recently received a nice note from John Machamer about his 1930 Kinner B-5 engine. The factory recommends an oil operating temperature of 122°F to 158°F during cruise conditions.
But John had a problem because his oil temperature ran at only 120°F.
Even after closing off one air inlet he could only get it up to 130°. While this is in the recommended range, it’s still very low. John is concerned he may not be getting proper lubrication in the rear gear cases, and was wondering if a multi-grade or single grade would be better for his engine.
Unfortunately, there is no clear answer here. The problem is that engines with exposed cylinders just have too much cooling air around the hottest part of the cylinder. This keeps the head and exhaust valve area cool, but does not put enough heat into the core engine.
As John pointed out, the engine does not have any oil circulation in the upper end. The pressurized oil only feeds the crank and master rod bearing, so it does not get very hot.
The concern here is not only improper lubrication for all parts of the crankcase, but also the possibility of rust and corrosion due to the engine oil not getting warm enough to boil off the moisture.
I have seen this problem before. With exposed cylinders, the cylinders stay cool. This was important back in the 1920s and 1930s because the octane of fuel was not that high.
When the exhaust valve and seat reached a certain point, it would cause pre-ignition, which would destroy the engine after a short time.
With exposed cylinders, they could keep the exhaust valve and seat cool enough to eliminate the pre-ignition problem and get the valve and seat to last.
Then as the octane of fuels increased, aircraft builders were able to run with higher engine temperatures, which led to higher performance and cowled engines.
The best oil to use
Now back to the original question: Which oil to use. I would go with an AD single grade oil, probably a grade 100.
Single grade oils tend to run a little hotter in an engine, and they hold up better in gears and drive systems.
The question on oil temperature is another problem. Although it would be nice to get the temperature up, these engines were not designed to run that hot.
If you could block off enough air to get the temperature up, you would cause other problems.
In the past, one of the answers to the problem was to use detergent oils containing ash to keep the rusting and corrosion under control.
The main reason that detergent oils are never used in aviation engines is that they cause pre-ignition from ash-type deposits. But with the really cool engine temperatures and relatively low compression ratio, pre-ignition was not a problem.
However most automotive oils contain zinc anti-wear additives that would attack silver master rod bearings.
To ensure against the zinc problems, some of the early radial engine manufacturers recommended the use of railroad engine oil. I have not been able to find where railroad oils were approved in this engine. But if you had access to a good library, you could find what oils were approved and one of them could be a railroad oil.